1. Field of the Invention
This invention relates to a control device for a cylinder, and more particularly to a control device for controlling lowering motion of a cylinder of a forklift truck or the like.
2. Description of the Prior Art
A conventional control device for a cylinder of such type is disclosed in U.S. Pat. No. 4,099,541 and generally constructed in a manner as shown in FIG. 1. More particularly, the conventional control device includes a spool type valve 1 which includes a spool 1a and is provided therein with a cylinder port 2. The cylinder port 2 of the spool type valve 1 is connected to a bottom-side chamber 3 of a cylinder C of a forklift truck. Also, in the cylinder port 2 is arranged an operation check valve 4 in such a manner that a poppet 4a of the operation check valve 4 defines a pilot chamber 5 therein. The pilot chamber 5 is provided therein with a spring 6, which acts to press against the poppet 4a with a seat 7. When the poppet 4a is pressed abutted against the seat 7, the cylinder port 2 is divided into a spool-side section 2a and a cylinder-side section 2b.
The poppet 4a is formed with an orifice 8, through which the cylinder-side section 2b is communicated with the pilot chamber 5. The pilot chamber 5 is constantly communicated with a pilot passage 9, which is closed with a pilot valve 10. More particularly, the pilot valve 10 includes a poppet 10a, which is pressed against a seat 12 by means of a spring 11, to thereby close the pilot passage 9. The so-constructed pilot valve 10 is adapted to be contacted at a tip end thereof with a tapered portion 13 formed at an intermediate section of the spool 1a. When the spool 1a is moved in a left direction in FIG. 1, the pilot valve 10 is raised along the tapered portion 13 of the spool 1a, to thereby open the seat 12. This results in the pilot passage 9 being communicated with a return passage 15 via a through-hole 14 formed in the pilot valve 10.
In the conventional control device constructed as described above, when the spool 1a is at a neutral position shown in FIG. 1, the pilot valve 10 is closed to keep the operation check valve 4a closed, so that load W of the cylinder C is kept at the position.
Then, when the spool 1a is moved from the position in a right direction in FIG. 1, the pilot valve 10 is kept closed. However, an inflow passage 16 is communicated with the spool-side section 2a of the cylinder port 2 to cause pressure fluid to flow through the inflow passage 16 to the spool-side section 2a. The pressure fluid then opens the operation check valve 4, resulting in being supplied to the bottom-side chamber 3 of the cylinder C.
On the contrary, when the spool 1a is moved from the position in a left direction in FIG. 1, the pilot valve 10 is raised along the tapered portion 13 of the spool 1a with the movement of the spool 1a to open the seat 12, resulting in the pilot passage 9 being communicated with the return passage 15. Such communication between the pilot passage 9 and the return passage 15 causes a difference in pressure to occur between both sides of the orifice 8 to lead to opening of the poppet 4a.
Further movement of the spool 1a in the left direction causes communication between the spool-side section 2a of the cylinder port 2 and the return passage 15 through an annular groove 17 of the spool 1a, so that fluid in the bottom-side chamber 3 of the cylinder C is returned from the cylinder port 2 through the return passage 15 to a tank to decrease the load of the cylinder C.
When a timing of opening of the pilot valve 10 is delayed, the operation check valve 4 is caused to open after communication between the cylinder port 2 and the return passage 15. Such delayed opening of the operation check valve 4 causes fluid to suddenly flow from the cylinder C to the return passage 15 concurrently with the opening of the valve 4 resulting in inching control and the like being difficult.
Accordingly, in the conventional control device, it is required to accurately determine a relative position between the pilot valve 10 and the tapered portion 13 of the spool 1a in order to prevent a timing of opening of the operation check valve from being delayed. Unfortunately, accurate determination of the relative position requires working the pilot valve and spool with high accuracy to cause an increase in the manufacturing cost of the device. Also, the conventional control device requires to separately arrange the pilot valve, so that the number of parts is significantly increased. This results in a further increase in the manufacturing cost.